This invention pertains to using an inhalation sensor for inhalation therapy and monitoring, and particularly using it with such apparatus and operating methods which features the triggering of a prescribed dose of therapeutic gas when inhalation takes place and/or detects when apnea (the absence of breathing) occurs.
There is an abundance of technology available for apnea detection. Apnea monitors using transthoracic impedance (requiring electrodes to be positioned on the chest) is presently the technique of choice for infants (ninety percent) and in adults (fifty percent), but it suffers from a fundamental deficiency: it provides a measure of thoracic effort rather than specific airway opening and patency.
The inhalation sensor, when used as an apnea monitor, overcomes this problem as it is measuring air flow at an airway opening such as the nasal openings or, if necessary, at the mouth.
The inhalation sensor must be capable of detecting movement of air at, or near, the mouth or nose. Thermistor temperature sensors can detect such air flow by detecting the cooling of a heated element. Air flow sensors are also marketed by such companies as the Micro Switch Division of Honeywell, and work on detecting air flow by cooling a heated element.
However, the problem with these devices is that they can not detect the direction of air flow and are incapable of indicating if the air is flowing in or out of the nose, and therefore, are not suitable for use as an inhalation sensor.
Pressure transducers that can detect extremely low pressures of 0.001 ounce per square inch and which are capable of detecting the vacuum created by inhalation and the pressure of exhalation, make excellent inhalation sensors. At the present time, semiconductor pressure transducers are of very low cost but are not capable of detecting negative and positive pressures of 0.001 ounce per square inch.
Pressure transducers designed for detection of such low pressures are often of the variable capacitance type as manufactured by MKS Instruments, Inc. of Burlington, Mass. . These transducers are all metal and are of high cost.
The lack of low cost air flow detection devices which use electric circuits has resulted in the use of fluidic devices to detect inhalation air flow.
In this regard, U.S. Pat. No. 4,457,303 to Durkon discusses the prior art and discloses the use of fluidics to obtain a device capable of detecting a negative pressure as small as 0.5 millimeters of water (approximately 0.01 ounce per square inch) by using a number of stages of fluidic amplification.
The applicant's inhalation sensor is capable of detecting negative pressure of 0.001 ounce per square inch by using a low cost non-metallic, metal-coated on one side diaphragm, variable capacitance type of pressure transducer, that is ten times more sensitive than the fluidic device described by Durkon.
A fundamental limitation to the application of pressure sensing or air flow detection by an inhalation sensor is keeping the inhalation sensor aligned with the airway exchange.
The applicant deals with this recognized difficulty by using the nasal cannula (which is an accepted method of administering oxygen for inhalation therapy) as a means of connecting a patient to the inhalation sensor.
Therefore, the physical positioning of the inhalation sensor is not important.
Often a patient, receiving oxygen via a cannula, will not have both prongs of the cannula properly aligned, and therefore, will not be receiving the benefits of the prescribed treatment. When this happens, the applicant's inhalation sensor can act as a monitor and sound an alarm and/or send a continuous flow of oxygen instead of a triggered dose.
The applicant's inhalation sensor, when used for inhalation therapy, supplies a dose of therapeutic gas each time a patient inhales. This means that since a patient inhales approximately 30% of the time, a possible saving of 70% of the oxygen used in a continuous flow system, can be saved. It also makes possible a greater volume of oxygen at an early stage of inspiration, and is more effective than conventional continuous flow because oxygen applied during the later stage of inspiration remains in "dead spaces" such as the pharynx, trachea, and bronchial tube. Oxygen given in the early stage of inspiration is most effective in reaching the alveoli.
This not only reduces the cost of oxygen, but eliminates a potential hazard by not having the 70% wasted oxygen being present in the environment.
If a patient breaths by shifting from nose to mouth, the cannula is replaced with a mouth/nose mask.
In prior art, two prongs were connected to two separate tubes, with one prong in one nasal opening being used as the sensor and the second prong placed in the other nasal opening, to supply the inhalation therapy gas.
The applicant's device requires only a single tube connected to the two prongs to serve as a sensor and supply the therapeutic gas.
The applicant's inhalation sensor requires no electrical connections to be made to a patient, and its electrical circuits can be made intrinsically safe (a device incapable of causing ignition of a flammable gas being used for inhalation therapy) because it is able to use low current and voltage for its operation.